1. Field of the Invention
The present invention relates to a ring oscillator and associated method.
2. Description of the Related Art
Circuits that require internal clocks may use a clock generator to generate periodic signals that may be used as the clock. Alternatively, the clock generator may produce periodic signals that are converted to slower clocks, using a counter or other means to divide the frequency. In yet another application, the clock generator may be used as a phase-locked loop for a clock generator for a memory device.
For example, dynamic memories require refresh signals to ensure that leakage in the memory cells does not cause the memory cells to lose their data. A clock generator may be used to generate a periodic refresh signal. In some examples, the periodic signal is fed to a counter, and after a predetermined number of counts, the counter may output a refresh signal.
One circuit that may be used to generate the periodic signal is a ring oscillator. Examples of these types of oscillators can be found in exemplary U.S. Pat. No. 6,100,763, issued on Aug. 8, 2000, and U.S. Pat. No. 5,250,914, issued Oct. 5, 1993. Generally, these approaches involve a single loop circuit of an odd-number of inverters. As the output signal is fed back to the inputs of the loop, the output signal is inverted, resulting in a signal that varies between a high and a low signal. This causes a periodic signal with a well-defined and stable cycle. The period of the output signal may be decreased, therefore increasing the frequency, by increasing the power in magnitude. This provides for an adjustable frequency.
In another approach, is found in S. J. Lee, “A Novel High-Speed Ring Oscillator for Multiphase Clock Generation using Negative Skewed Delay Scheme,” IEEE Journal of Solid-State Circuits, February 1997, pp. 289–291. Lee uses a skewed delay scheme to employ signals of differing phases to generate a multiphase signal. However, the approach does not generate a signal that provides a much faster signal than the prior art.
As memory and other technology evolves, a faster oscillating clock signal is needed for such tasks as refresh signals, system clocks, phase-locked loops, etc. The above solutions, as well as others in the current state of the art, do not provide signals that have a high enough frequency to keep up with the emerging circuit technologies.